I make so many mistakes in my own posts, I know the difference between 'their' and 'they're', 'of' and 'have' and they'll still pop up in my posts. Not in my written work so I put it down to speed and not concentrating because I'm on a chatboard. I've never seen anybody say that SPaG isn't important, not anywhere, it is but, as CanterburyWhales said, it's a chatboard, not a literacy lesson.
My last post has a fundamental error on it, it should read 'by the posters who use SPaG transgressions as an excuse', not 'of'. But, the objective is for people to understand and context is everything.
Every single time I see one of these threads I imagine an OP who isn't held in particularly high regard by the people that matter to them, feel that they aren't listened to, haven't achieved what the wanted to - or some combination. I think that because it takes some kind of special impetus for somebody to launch a diatribe about something they perceive as lacking that many people can't help. That they themselves make mistakes in their posts, they ignore, but feel that other people are fair game. That's something very lacking indeed, but not of the posters 'committing' SPaG atrocities.
The fact that Jocasta didn't understand why a doctor didn't correct a patient referring to 'prostrate' when they meant 'prostate' speaks volumes. So little understanding of the fact that well-rounded and emotionally intelligent people don't need to 'red pen' their way through life, picking up people on their errors all the time.
Firsttimelottie made a great suggestion about the OP teaching the correct meanings/context of words instead of mocking. Unsurprising that there are no takers for that. Says so much that making people feel worse for their written skills is more desirable. If there really were concern about SPaG 'failures', then what more opportunity? But it's not the case and never was, it's the 'joy' of pointing and laughing.
I am invested in this and I loath seeing cowardly threads like this where the intention is clear however much an OP wants to cloak it. I'm glad that I'm not on my own there.